Vibratile sensing instrument

ABSTRACT

An instrument for the detection or measurement of attributes of a material comprises at least one vane which is mounted on a support that allows the vane to vibrate, a drive transducer which is disposed relative to the vane to stimulate vibration therein and at least one sensing transducer disposed to sense the vibrations.

BACKGROUND OF THE INVENTION

This invention relates to the detection or measurement of attributes ofmaterials generally, for example density, flow rate, moisture content orother attributes or parameters of flowable materials, such as gaseous orliquid fluids, emulsions, particulate or granulate materials orsuspensions of solids in liquids, such as a slurry.

A wide variety of devices for such detection and measurement are knownin the art. Most of them are specific to a particular attribute orparameter. The general object of the present invention is to provide areliable and versatile device which can generally be used for ameasurement of a variety of different attributes.

The state of the art is represented by GB-A-2222683, which describes asensor that externally resembles a preferred embodiment of the inventionbut operates rather differently. The sensor described in GB-A-2222683comprises an elongate support, preferably in the form of a shaftcarrying, preferably near one end, a plurality of laterally extendingfins or vanes constituting capacitor plates. The vanes protrudelaterally from the support but are generally elongate. The sensorincludes circuits responsive to the capacitance between pairs of platesin order to derive an indication of a property, such as moisturecontent, of the material which is disposed between the plates or finswhen the sensor is immersed in that material.

A capacitive technique, like most measurements of moisture, isessentially a measurement of two volumetric ratios. However mostdefinitions of moisture content are by ratio of mass. Although, forexample, water density over a wide temperature range is well known, itis also necessary to know the bulk density of the true moisture contentby mass to be measured.

The present invention is based on the exploitation of the fact that whena fin or vane is stimulated into small vibrations it has degrees offreedom in two planes, along and across the vane. It obeys a secondorder rave equation resulting in standing waves set up along the vane.If such a vane is immersed in a flowable material, it is found that thesquare root of the frequency of oscillation is inversely related to thedensity of the surrounding medium. However, analysis of wave equationsolutions indicates that there is a much greater number of resonantmodes and frequencies than are present in, for example, a vibrating tubedensitometer. Accordingly, the resonant properties allow exploitationbeyond the mere scope of density measurement.

Broadly, according to the invention, an instrument for the detection ormeasurement of attributes of a material comprises at least one vanewhich is mounted on a support that allows the vane to vibrate, a drivetransducer which is disposed relative to the vane to stimulate vibrationtherein and at least one sensing transducer disposed to sense saidvibrations.

In preferred embodiments of the invention the support is elongate. Thevane may comprise a fin which protrudes laterally from the support.However, many other configurations are feasible.

Where the vane is elongate, the said transducers may be spaced apartalong the length of the vane. A sensing transducer may be disposed atsubstantially an antinode of the vibration of the vane. The support mayengage the vane at at least one region corresponding to a node of thevibration. In particular, there may be means for clamping the vane neara median region and the drive transducer may be disposed to induce avibration in a plane transverse the median line.

At least one of the transducers may be disposed on a peninsular portionwithin the vane. The peninsular portion may be formed as a partly cutouttab which lies in the local plane of the vane. The vane may be clampedbetween such a peninsular portion and an adjacent part of the vane sothat the peninsular portions and adjacent portions each constitute acantilever. This is useful both for the induction of vibration in andthe sensing of vibrations in the vane. The said one transducer may be adrive transducer and the peninsular portion on which the drivetransducer is disposed may carry a substantial mass. Where the vane isclamped near the median region, the various transducers may each bedisposed on a respective peninsular portion positioned in the medianregion and spaced apart along it.

By itself a drive transducer might be insufficient to induce vibrationsof sufficient amplitude in the vane, and accordingly it is generallydesirable to provide a regenerative or positive feedback coupling fromthe sensing transducer, or one of the sensing transducers, to the drivetransducer. This coupling may include a band-pass filter for therejection of signals other than those associated with a principal ordesired mode of vibration of the vane.

As will be explained hereinafter, a useful output may be represented bythe output frequency of a sensing transducer or may be obtained bycomparing the phase of signals at the drive transducer and one of thesensing transducers or may be obtained by comparison of the amplitude,frequency or phase of the outputs of two sensing transducers disposed atdifferent positions on the vane, for example at antinodes of vibrationthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates sectionally an embodiment of the invention in partlyassembled form;

FIG. 2 illustrates a vane forming part of the embodiment shown in FIG.1;

FIG. 3 illustrates a constructional detail of the embodiment of FIG. 1;

FIG. 4 illustrates an electrical circuit of the detail shown in FIG. 3;

FIG. 5 illustrates a lateral cross-section through the embodimentillustrated in the previous Figures.;

FIGS. 6A, 6B and 6C are explanatory diagrams indicating the inducing andsensing of oscillations in a vane by the particular embodimentillustrated in FIGS. 1 and 5;

FIG. 7 is an electrical schematic diagram of a processing circuitincluding drive transducers and sensing transducers provided in theembodiment of FIG. 1; and

FIG. 8 illustrates in simplified form a simplified but typical frequencyresponse of the device shown in FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The device or sensor shown principally in FIGS. 1, 2 and 5 includes asupport 1 which may be in the form of an elongate body which may have apointed or tapering leading end 1a so that the device is adapted, ifdesired, for insertion into flowable material. However, otherconfigurations of the body are possible. The support may be streamlinedwhere the device is intended for use in a fluid flow.

The support 1 supports at least one vane which in this embodiment of theinvention is configured to provide a pair of fins which protrudelaterally from the support. The vane 2 is held in place within thesupport 1 by means of a clamp member 3 which fits into and constitutespart of the support. The support 1 has a longitudinal recess 5. Therecess 5 has two longitudinal side walls 5a and 5b and end shoulders 6aand 6b so that the vane is in this embodiment of the invention clampedby the clamp member 3 along two lines extending parallel and adjacent toa median line of the vane.

The clamp member 3 has flat underportions 7, 8, 9 and 10 which engagethe shoulders 6a, 6b and the sidewall portions 5a and 5b so as not onlyto clamp the vane but also to define an interior recess in which varioustransducers and electrical connections to them are disposed. The clampmember 3 may be bolted, welded or otherwise secured to the support 1 byany suitable means, releasable or otherwise, and an appropriate seal maybe made between the clamping block 3 and the support 1. The clamp member3 has recesses 10, 11 and 12 spaced apart along its length where theclamp member is disposed over the positions of the transducers.

In this embodiment of the invention the vane 2 is symmetrical about botha longitudinal median line and about a transverse median line, having asshown in FIG. 2 two parallel longer sides 13 and 14 and two symmetricalend sides 15 and 16. The vane may be made of steel or other suitableself supporting elastic material or combination of materials.

However, other configurations for the vane are possible. In particular,the vane may be clamped along and adjacent a side margin. However, suchvariants will not be described in detail.

As indicated previously, the vane is to be stimulated into vibration. Avane has two degrees of freedom, along and across the vane. Thestimulation of the vane into vibration may be performed by at least onetransducer, such as the transducer 20. Also, at least one sensingtransducer, such as the transducers 21 and 22, are provided for sensingthe vibrations of the vane. Though the positioning of the drivetransducer and the sensing transducer or transducers depends largely onthe configuration of the vane and the desired modes of vibration, it isnormally preferable to dispose the transducers at antinodes of thedominant standing wave pattern of vibration. A preferred configurationfor the present embodiment is the disposition of the transducers spacedapart along the length of the vane, preferably in the longitudinalmedian region thereof. In particular, as shown in FIG. 2, the drivetransducer is disposed centrally of the vane and each of the sensingtransducers is disposed longitudinally spaced apart from the drivetransducer.

FIG. 2 illustrates particularly a preferred construction for themounting of the transducers relative to the vane in order to provide anefficient means of inducing vibration in the vane and for sensingvibrations in it. In particular, the drive transducer 20 is disposed ona peninsular portion 17 which is formed within the vane. This peninsularportion 17 may be formed as a partly cutout flap which is therebyseparated from the surrounding vane along three sides of aquadrilateral, the remaining side forming an isthmus joining thepeninsular portion to the surrounding vane. The peninsular portion orflap may be of any suitable shape; the flap 17 for the drive transducerhas the form of a symmetrical trapezium. Similar, square, peninsularportions, i.e., flaps 18 and 19 are provided for the mounting of thetransducers 21 and 22. The transducers may be piezoelectric but otherforms of transducer can be employed.

Each transducer has attached to it a fine wire 23 which may be fed alonga channel in the body and through a hole in the end of the support 1whence it can then be connected to the external circuitry.

It is convenient to provide a buffer or preamplifier within the body ofthe instrument so as to improve the signal to noise ratio of thecomparatively weak signals obtained from a sensing transducer before thesignals are conveyed to an external circuit but in other configurationsof the device this feature may not be required or appropriate. As shownin FIG. 3 wherein a field effect transistor 24 is disposed on theunderside of the vane 2, a connecting wire 25 extends from thetransducer 22 to the gate of the transistor. That gate is also connectedto the vane 2 by way of a bias resistor 26, one lead of which isphysically and electrically secured to the vane 2 by solder 27. Thesource and drain of the transistor are connected by way of leads 28 and29 to the external circuit.

FIG. 4 illustrates schematically the electrical network formed by thetransducer 22 and the associated field effect transistor 24. In thisconfiguration the vane 2 acts as an electrical earth.

FIG. 5 shows a cross-section through the sensor and illustrates theconfiguration of the vane 2 as two portions (conveniently termed fins)2a and 2b, which in this embodiment extend laterally one to each side ofthe support 1.

FIG. 5 also illustrates the configuration of a flap relative to the vaneand the clamping of the vane adjacent the isthmus of the flap.

As shown in FIG. 5, and also in FIG. 1, the flap 17 that carries, on oneside, the drive transducer 20 also carries a substantial mass 4. Theutility of this mass, which depends on the configuration and otherphysical attributes of the vane, will be explained with reference toFIGS. 6A to 6C.

FIG. 6A is a schematic diagram showing the mass 4 mounted on the flap17, the portions 2a and 2b which are separated from each other along aline extending along the flap or peninsular portion, but are otherwiseinterconnected, clamps 3a and 3b adjacent each end of the peninsularportion. These clamps are constituted by the wall portions 5a and 5b ofthe support 1 and the adjacent parts of the clamping member 3.

The flap 17 forms by virtue of the clamp 3a a cantilever system with theadjacent portion 2a of the vane 2.

FIG. 6A illustrates the initial phase of oscillation of the vane. Thedrive transducer, omitted for convenience from FIG. 6A, is excited by asuitable, preferably sinusoidal, alternating signal. This excites themass which is bonded to the flap, as shown by the double-headed arrow A.Since the flap is effectively a cantilever beam, the movement of thebeam causes a reaction at the clamped part of the beam, that is to saywhere the vane is clamped to the support. However, because the body inrelatively massive and rigid it cannot move in reaction to the vibratingcantilevered mass. Instead, the external portion 2a of the vane, whichis free to move, is excited into oscillation, as shown by thedouble-headed arrow B. The greater the mass 4, the greater is the torquecreated at the clamp and the greater the corresponding amplitude ofvibration of the vane.

Because the external portions 2a and 2b of the vane on each side of thebody of similar mass and shape, they form a mechanically balancedresonant system. Stimulation of one fin or vane portion 2a causes acorresponding reaction in the other. If therefore the internal mass isexcited at a resonant frequency of the system of fins then the fins willbe excited into vibration of maximum amplitude.

Thus in FIG. 6B, the double-headed arrow C shows the vibration of theportion 2b.

FIG. 6C shows the stimulation of a pickup flap, this being the converseof the stimulation process previously described. Movement of theexternal fin 2b, as shown by the double-headed arrow C, causes areaction at the clamp 3b which, stimulates the pickup flap 18 intovibration, as shown by the double-headed arrow D.

It is obviously preferable for the peninsular portions or flaps to be aslight as possible so that they require only an insignificant amount ofenergy for their vibration.

FIG. 7 illustrates by way of example an external electrical circuit foruse with the instrument; reference will first be made to FIG. 8, whichillustrates a typical frequency characteristic 80 of the sensor, whichis actually constituted by a set of cantilevered beams with their ownresonant frequencies. FIG. 8 illustrates a resonant peak 81corresponding to the resonant frequency of the driver flap 17, resonantpeaks 82 and 83 corresponding to the resonant frequencies of the pickupflaps and other resonant peaks 84, 85, 86 and 87 corresponding to theresonant frequencies in the various modes of vibration of the vane. Thefrequency characteristic shown in FIG. 8 is particular to the systemshown in FIGS. 1, 2 and 5. Obviously, for a different configuration ofthe vane there may be fewer resonant peaks or more resonant peaks.

It is in general feasible to construct the instrument so that theindividual flap frequencies 81 to 83 are in a frequency range which doesnot overlap with the range in which fall the resonant frequencies of theparts of the vane. A similar effect can be produced by the addition ofweights on the flaps to alter their resonance.

The resonant frequencies of the two fins 2a and 2b are, for similarmodes, normally so close that they appear as one frequency.

There are various ways of driving the instrument. Preferably however theinstrument is driven by means of a regenerative amplifier system, whichis illustrated by way of example in FIG. 7.

Referring now particularly to FIG. 7, the sensing transducer 21 iscoupled to an input amplifier 71. The output of this amplifier isavailable at a terminal 72 for use as hereinafter explained. The outputof the amplifier 71 is fed through a band-pass filter 73 which may beadjustable. The pass band 88 of the filter is shown in FIG. 8 ascentered on the desired resonant frequency so as to providediscrimination against all other known signals from the vane or thedrive and the pickup flaps.

The output from the band-pass filter may then be adjusted for phasediscrepancy by means of a phase adjuster 74. For resonance, the drivesignal should be in phase with the signal at the pickup 21,

Since overdriving of this system can introduce unwanted harmonics, it ispreferable to provide an automatic gain control circuit 75 limiting thegain to a value set by a control 76. The output from the automatic gaincontrol circuit is fed by way of an output amplifier 77 to the drivetransducer to complete the regenerative loop. The output of the AGCcircuit 75 may also be coupled by way of a limiting amplifier 78 whichprovides an output indicating the frequency of the oscillating system.

The other sensing transducer 22a provides an output which is coupled byway of a respective input amplifier 79 to a respective terminal, so asto enable a phase comparison to be made between signals from the sensingtransducers, representing the vibrations at the two ends of the vane or,in general, to different portions thereof.

These outputs may be used for various purposes, depending on the mode ofuse of the instrument. Some of these uses are described in thefollowing.

For example, the instrument may be used to measure the density of aflowable medium in which it is immersed. The resonant frequency ofvibration is given by the expression:

    f.sub.0 =k.sub.1 √{[k.sub.2 (1-αδt)]/(r+M.sub.0)}

where f₀ is the resonant frequency at temperature t;

α is the thermoelastic coefficient of the material of the vane;

δt is the difference between the ambient temperature and a calibrationtemperature;

r is a parameter of the density of the surrounding medium, in units ofmass per unit length;

M₀ is a parameter of the mass of the vane, also in units of mass perunit length; and

k₁ and k₂ are calibration constants at a calibrated temperature.

The parameter r, and thence the density, can be deduced if desired, by aprocessing circuit, by solving equation (1) if all elements of theequation are known. In practice it is possible to calibrate theinstruments so that only a measurement of frequency is required.

The elastic properties of nearly all materials change with temperatureand this affects the resonant frequency as shown by the aboveexpression. However, by measuring the shift in frequency of one of theinternal cantilever flaps with temperature the elasticity shift it canbe deduced.

Because the mass and surrounding density of an internal flap areconstant, the shift in frequency with temperature can be measured fairlyeasily. The resonant frequency of a flap for the sensor 21 may bemeasured by shifting the pass band of the pass-band filter to the regionof the resonant frequency, this being measured as some calibrationtemperature. This frequency is expressed as F_(cal). Thereafter when theinstrument is being used, the control circuitry may occasionally checkthe frequency F of the flap. Any deviation from the calibrationfrequency can be attributed to a temperature shift from the calibrationtemperature T_(cal). The ratio of the shift in these frequencies is thesame as the shift of the frequencies of their fins from their calibratedtemperature, so that Fo_(cal) =f_(t) F_(cal) /F_(t), where Fo_(cal) isthe frequency of vibration of the vane corrected to the calibrationtemperature, f_(t) is the frequency of vibration of the vane at theactual temperature t, F_(cal) is the frequency of the flap at thecalibration temperature and F_(t) is the frequency of flap at the actualtemperature t.

F_(cal) is measured at the calibration stage; F_(t) and f_(t) aremeasured during the use of the instrument. The switching of theband-pass filter to measure the flap frequency can be controlledautomatically by a microprocessor.

If the fins are damaged or worn, their mass may change. This will changethe characteristics of the instrument, causing it to go out ofcalibration. It is possible to detect changes in the mass parameter M₀by two methods.

According to one method, if one fin wears more than the other then twodiscrete resonant peaks will appear in the frequency characteristic, onefor each fin. One peak will be out of phase with the other. By adjustingthe phase of the drive signal it is possible to measure the resonance ofthe fins individually, the difference in the two frequencies beingproportional to the difference in mass.

In a second method, if wear occurs equally on both fins there will be noapparent shift in relative resonant frequencies as described in thefirst method. However, wear will normally occur on the leading edge of afin. Normally the two sensor pickups will provide in phase signals butif wear occurs near the leading edge of the fin then the effective massthere is reduced, causing an inbalance in the vibration. This manifestsitself by a change in phase of the pickup signals as between the ends ofthe fins. The difference in phase is proportional to the difference inmass along the fin.

Resonant densitometers require the surrounding medium to be Newtonian,that is to say to possess a degree of elasticity or viscosity. It ispossible to measure the elasticity or viscosity of a liquid by switchingresonant frequencies and measuring the relative amplitudes of tworesonant peaks. The use of a vibrating vane permits measurement in othermodes. For example, the viscosity of the fluid surrounding the warheadpresents a damping factor which reduces resonant amplitude and slightlyalters the frequency. Thus, if the damping factor C=μk₁ where k₁ is aproportional constant and μ is the dynamic viscosity, it can be shownthat the amplitude X at resonance is equal to Fo/CΩ_(n), where C is thedamping factor, Fo is the drive signal amplitude, normally constant, andΩ_(n) is the resonant frequency. Thus, one can write C=k₂ /XΩ_(n), wherek₂ is a constant or μ is equal to k₃ /XΩ_(n), where k₃ is a differentconstant.

The constant k₃ may actually shift with time due to wear, stress orother factors. These effects can be much reduced by measuring therelative amplitudes of two different frequencies. This results in theexpression:

    μ=k.sub.CAL (1/Ω.sub.n1 -1/Ω.sub.n2)/(X.sub.1 -X.sub.2)

where k_(CAL) is a constant of proportionality, Ω_(n2) and Ω_(n2) aretwo distinct resonant frequencies and X₁, X₂ are the correspondingamplitudes of vibration at those frequencies.

One particular problem especially in the offshore oil industry, is theassessment of mixture ratio. It particular applies to the determinationof the quantities of natural gas liquids and water entrained in crudeoil pipelines. The water content can be estimated from previouslydescribed techniques but natural gas liquids have proved almostimpossible to monitor. The general measurement is called three-phasemeasurement because the water can be presumed to carry sediment, thoughthis is normally removed by filters.

The viscosity of crude oil is made less by the presence of lightercomponents such as natural gas liquids. For example, the viscosity ofcrude oil is approximately 25 centi-poises whereas the viscosity ofN-Pentane is approximately 0.2 centi-poises.

Using a vibrating vane as described one may measure the viscosities ofcrude oil (μ_(CRUDE)) and a mixture of crude oil and natural gas liquid(μ_(MIX)) and using a known or determined measure of the viscosity ofthe natural gas liquid (μ_(NGL)) obtain the mixture ratio from theexpression:

    (μ.sub.MIX -μ.sub.NGL)/(μ.sub.CRUDE -μ.sub.NGL)

The viscosity of a liquid in which the instrument is immersed createsviscous damping which reduces the amplitude of a resonant peak.Measurement of the amplitude gives a measure of the damping which may becollated with the dynamic viscosity. Measuring the relative amplitude oftwo peaks improves the measurement by reducing the effect of long termchanges in the characteristics of the sensor.

As the viscosity of the measured fluid increases, the damping effectcauses a decrease in resonant frequency determined by:

    F.sub.n =F.sub.o √(1-z.sup.2)

where F_(n) is the damped frequency, F_(o) is the undamped frequency andz is the damping factor.

The damping factor may be determined by the viscosity which is measuredas described previously. From the measured damped frequency the undampedfrequency can be derived and applied to the density equation.

Where the damping factor of the material surrounding the fins is sogreat that frequency measurement by self oscillation cannot be sustainedthen the resonant frequency can be determined by examining the phaserelationship between the drive and the output signal from a sensingtransducer. This is given as:

    tan θ=2z(f/f.sub.0)/[1-(f/f.sub.0).sup.2 ].

At resonance the phase angle is 90° irrespective of the damping factorin the numerator. Observance of the frequency at quadrature willindicate resonance. This technique may also be used in circumstances oflow viscosity as an alternative to regenerative driving.

Other attributes can also be measured by the instrument directly orindirectly. For example, if a dry medium has a known density which issignificantly different to that of water, for example most oils, thenthe water content can be calculated from the line density using theequation:

    M.sub.m =R.sub.w (R.sub.b -R.sub.d)/R.sub.b (R.sub.w -R.sub.d)

Where R_(w) is the water density, R_(b) is the measured bulk density andR_(d) is the dry density.

Some liquids exhibit a constant thermal expansion factor withtemperature: this is the case for many oils. However, the temperaturecoefficient of water is particularly non-linear. By monitoring changesin temperature and density over a period and then differentiating withrespect to temperature, the second differential yields the watercontent:

    M.sub.v =(δ.sup.2 R.sub.b /δt.sup.2)/(δ.sup.2 R.sub.w /δt.sup.2)

The invention could be incorporated into a sensor as described inGB-A-2222683 to enable the measurements made to be augmented by orcorrelated with capacitative measurements.

A further development of the invention concerns the measurement of flowrate by cross correlation. In particular, two instruments may bedisposed in tandem some distance apart. In an environment where themeasured attribute is changing rapidly the output waveforms from onesensor will be delayed relative to output waveforms from the other by atime T which depends on the flow rate. Using a cross correlationfunction the time T can be estimated and, because the distance is known,the flow rate may be found.

This function can also be applied to the density output. The densitysensor can operate at significantly higher frequencies to other densitytransducers, for example it may operate at 12 kilohertz. The responsetime is inversely proportional to frequency so density signal responsesof the order of 100 microseconds are possible.

Also possible is a further arrangement in which two instruments inseries or tandem provide a measurement of mass/volume flowrate byfrequency shift. Suppose two instruments each with a vibrating vane aredisposed in series or one after the other so that one vane resonates atits natural frequency and the waves produced by the one vane activatesthe vane of the second instrument. If the two vanes are, for example,physically identical, they will resonate at approximately the samefrequency thereby creating a tuned system. If a flowrate in thesurrounding medium is introduced, the transmitted waves will experiencea doppler shift at the receiver vane. The difference between thefrequency transmitted by the transmitter vane and that produced by thereceiver vane is approximately proportional to flow rate. Furthermore,there will be a phase difference between the transmitted and receivedsignals also proportional to flowrate. This is more likely to bedetectable under very low flow conditions.

The multiplication of volume flow rate by the density will yield massflowrate.

I claim:
 1. An instrument for the measurement of attributes of a material, comprising:a support; an elongate vane which is symmetrical about a longitudinal median line; means for clamping the vane to said support in a region extending along said longitudinal median line wherein said vane constitutes two similar fins in a mechanically balanced resonant system; a drive transducer which is disposed to induce in said vane vibration in a plane transverse said longitudinal median line; and at least one sensing transducer disposed to sense said vibration.
 2. An instrument according to claim 1 wherein said drive transducer and said sensing transducer are each positioned in the region of the median line and are spaced apart along said median line.
 3. An instrument according to claim 2 wherein at least one of said drive transducer and said sensing transducer is mounted on a peninsular portion within said vane.
 4. An instrument according to claim 3 wherein said means for clamping clamps said vane at a position between said peninsular portion and an adjacent portion of said vane, said peninsular portion and said adjacent portion each constituting a cantilever.
 5. An instrument according to claim 3 wherein said peninsular portion carries a substantial mass.
 6. An instrument for the measurement of attributes of material, comprising:a support; a vane mounted on said support, said vane being symmetrical about a longitudinal median of said vane; means for clamping said vane to said support along said longitudinal median wherein said vane constitutes two similar fins in a mechanically balanced resonant system; a drive transducer disposed to engage said vane for inducing vibration in said vane; means for mounting said drive transducer to enable said transducer to induce said vibration selectively in a plane transverse said longitudinal median; and a sensing transducer disposed to engage said vane to sense said vibration.
 7. An instrument according to claim 6 wherein said drive transducer and said sensing transducer are disposed on and spaced apart along said longitudinal median.
 8. An instrument according to claim 6 wherein said means for mounting comprises a respective peninsular flap within said vane, said peninsular flap and an adjacent portion of said vane forming a cantilever system with each other.
 9. An instrument according to claim 8 wherein said flap carries a substantial mass in addition to said drive transducer.
 10. An instrument for the determination of attributes of a medium, comprising:a support; at least one vane mounted on said support; a drive transducer which is disposed relative to said vane to stimulate vibration therein; at least one sensing transducer disposed to sense said vibration; and a peninsular flap formed within said vane and constituting with an adjacent portion of said vane a cantilever system, said drive transducer being carried on said flap.
 11. An instrument according to claim 10 wherein said flap also carries a substantial mass.
 12. An instrument according to claim 11 wherein said vane is symmetrical about a longitudinal median, said drive transducer being disposed at a position along said median.
 13. An instrument according to claim 12 and further comprising means for clamping said vane to said support along said median.
 14. An instrument according to claim 13 wherein a second peninsular flap is formed within said vane, said second peninsular flap carrying said sensing transducer.
 15. An instrument according to claim 14 wherein said drive transducer and said sensing transducer are spaced apart said longitudinal median. 